linux-kernel - Re: [PATCH 2/3] mm: calculate deferred pages after skipping mirrored memory

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Message-ID: <CAGM2reb9CLpq1cqPLVqYXEfUqBtCt4V0OL_F=CKCyJXqV88NcQ@mail.gmail.com>
Date:   Tue, 24 Jul 2018 21:19:11 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kirill.shutemov@...ux.intel.com, Michal Hocko <mhocko@...e.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        dan.j.williams@...el.com, jack@...e.cz, jglisse@...hat.com,
        Souptick Joarder <jrdr.linux@...il.com>, bhe@...hat.com,
        gregkh@...uxfoundation.org, Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...il.com>, dave.hansen@...el.com,
        rientjes@...gle.com, mingo@...nel.org,
        osalvador@...hadventures.net, abdhalee@...ux.vnet.ibm.com,
        mpe@...erman.id.au
Subject: Re: [PATCH 2/3] mm: calculate deferred pages after skipping mirrored memory

On Tue, Jul 24, 2018 at 9:12 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Tue, 24 Jul 2018 19:55:19 -0400 Pavel Tatashin <pasha.tatashin@...cle.com> wrote:
>
> > update_defer_init() should be called only when struct page is about to be
> > initialized. Because it counts number of initialized struct pages, but
> > there we may skip struct pages if there is some mirrored memory.
>
> What are the runtime effects of this error?

I found this bug by reading the code. The effect is that fewer than
expected struct pages are initialized early in boot, and it is
possible that in some corner cases we may fail to boot when mirrored
pages are used. The deferred on demand code should somewhat mitigate
this. But, this still brings some inconsistencies compared to when
booting without mirrored pages, so it is better to fix.

Pavel